A lithium-ion battery may be summarily defined as follows: the battery comprises a cathode material (for example LiFePO4, LiCoO2, FeS2, V2O5, etc.), a lithium salt (for example LiPF6, LiTFSI, LiClO4, LMO, Li2CO3, etc.) dissolved in a liquid solvent or a polymer, and an anode material (for example graphite, LTO, etc.). In cases where the anode and/or cathode materials are not good current conductor, the material can be coated with carbon and/or be deposited on metallic substrates (for example aluminum, copper, etc.).
FIG. 1 outlines the operation of a lithium-ion battery. Reference numeral 10 depicts the copper cathode current collector, reference numeral 11 depicts the lithium ions conductive electrolyte, and reference numeral 12 depicts the aluminum anode current collector. During operation of the battery, oxidation of the anode material leads to de-intercalation of lithium ions, and simultaneously the cathode material undergoes a reduction reaction leading to intercalation of the lithium ions into its structure. Thereafter, the battery may be charged by application of an external current. The external circuit creates movement of the electrons from the cathode (which is in a reduced state) towards the anode. This leads to oxidation of the cathode material and thus de-lithiation restoring the lithium in the anode material. Following this process, the battery may be charged and discharged in a thousand cycles.
A dye-sensitized solar cell (DSSC) may be summarily defined as follows: the system requires that at least one of its faces comprise a current collector which is transparent to light (FIG. 2, arrow 1′). The transparent current collector can be a metallic grid or a very thin layer of a metal, a conductive polymer or a transparent substrate (glass or polymer) coated with a layer of a material which is transparent and conductive such as an oxide (for example FTO, ITO, Al-doped ZnO, Ga and/or Si, etc.), a conductive polymer (for example PEDOT:PSS, etc.) or metallic grids.
The photosensitive layer in a DSSC (FIG. 2, arrow 2′) comprises a layer of a semiconductive material (for example TiO2, ZnO, SnO2, “core-shell”, etc.). The layer must be as much transparent as possible and must enable the adsorption of the photosensitive dye. Typically, the photosensitive dye comprises organometallic molecules. This includes dyes wherein molecules have pyridyl groups and ruthenium (for example industrial dyes known as “N3”, “black dye”, “SJW-E1”, “N719”, etc.). The photosensitive dye may also comprise organic molecules only (for example “TA-St-CA”, etc.).
The electrolyte in a DSSC (FIG. 2, arrow 3′) may be a liquid, a gel or a solid. In any case, the electrolyte must comprise a sacrificial redox couple. Typically, the sacrificial redox couple is I3−/I−. However, other redox couples may also be used (for example Br3−/Br, SeCN−/(SeCN)2, (SCN)2/SCN−, Co3+/Co2+, etc.). A catalyst (for example platinum, gold etc.) is generally used (FIG. 2, arrow 4′) in order to increase the recombination speed of the sacrificial couple.
Finally, a DSSC generally comprises a current collector (FIG. 2, arrow 5′). The current collector may be transparent such as the one illustrated in Scheme, arrow 1′, or non-transparent.
FIG. 3 succinctly outlines the operation of a DSSC. Reference numeral 13 depicts a semiconductor, reference numeral 14 depicts a dye, reference numeral 15 depicts the electrolyte, reference numeral 16 depicts a counter-electrode made of conductive glass, and reference numeral 17 depicts the external circuit.
In a DSSC, the flux of electrons is created by the excitation of the photosensitive dye. Excitation is effected by light and by the fact that the lowest unoccupied molecular orbital (LUMO) of the dye has an energy level higher than the energy level of the conduction band of the semiconductor. Accordingly, electrons may be captured by the semiconductor and then the current collector when they leave the excited dye (S*). The dye is oxidized into S+ and immediately reacts with the sacrificial redox couple R/R− according to the reaction S++R−→S+R. Finally, the electron arriving at the counter-electrode through the external circuit serves in the recombination of the sacrificial redox couple. Given that the reactions occurring are governed by kinetics, electron extraction from the excited dye via the semiconductor then the current collector must be faster than the natural relaxation of the dye, in order to obtain this reaction mechanism.
Extensive research aimed at improving the quality of batteries is being conducted. A large part of this work relates to electrodes.